It is well known that cryogenic liquids, or liquids having similar properties, have found great use in industrial refrigeration and freezing, cryo-biological storage repository and lab test applications. Cryogenic liquids are typically stored in thermally insulated bulk tanks which consist of an inner vessel mounted inside, and thermally isolated from, an outer vessel. The liquid is then directed from the tank through thermally isolated pipes to a supply point where it is used for a variety of applications such as industrial, medical, or food processing.
Prior art bulk tanks typically use a pressure regulator at the top of the bulk tank. Such a system is limited in its flexibility. When the tank is full there is a certain amount of liquid head pressure. This head pressure is added to the tank vapor pressure and this is the supply pressure out of the tank. For some applications it may be important to maintain a constant supply pressure. As the liquid level in the tank drops from usage the vapor pressure in the tank needs to increase to compensate for the decrease in head pressure.
A mechanical pressure regulator is set to open when the pressure in the bulk tank drops below a set point and closes when it rises above the set point. The regulator is usually set to provide enough pressure inside the tank to operate at low liquid levels. This means that the supply pressure will be higher when the tank is full and drop off as the liquid level drops. As a result, a user may experience product losses or loss in efficiency near the bottom of the tank. This is not ideal for high flow rates where the condition of the supplied cryogenic liquid is important.
Failure to install a properly designed system for storing and dispensing cryogenic liquid with consistent quality causes wasted energy in lost cooling power. The poor control of the liquid conditions allows the outlet pressure to fluctuate so wildly that many times customers cannot utilize the lower one-third of the tank's capacity. The primary culprit of this complaint stems from a reduction in tank outlet pressure (tank vapor+liquid head pressure) at the liquid withdrawal point. This leads to a reduction in liquid flow rate at the application and as a result, inconsistent cooling.
In applications such as food freezing where the product is moving at a specified rate in the tunnel, it's critical that the quality of the cryogenic liquid being dispensed is consistent so the process can be tuned for maximum production throughput. If it becomes out of tune from liquid conditions changing at the application, the only recourse a plant manager has control over (other than slowing down production) is to call their liquid supplier and expedite the tank refill in order to restore the liquid to pre-tuned conditions. Not only is this an emergency delivery, but it's usually before the desired refill point so the tank can't take a full trailer load. The fresh liquid resolves the problem because it is usually colder and lowers the overall liquid saturation pressure, but more importantly, the pressure at the bottom of the tank is increased so the tuned liquid nitrogen flow rate is restored. A simple electrical analogy is like a voltage outage has just been restored. The cryogenic food freezer, like any electrical appliance wants to run on a constant supply pressure or voltage, so the liquid nitrogen flow rate or amperage draw remains constant.
A need therefore exists for a bulk cryogenic liquid pressurized dispensing system and method that addresses the above issues.